The present invention relates to a method for controlling the operation of a switching device. More particularly the present invention relates to a method for controlling the opening/closing operation of a switching device disposed on an electrical network, which is connected to an inductive component, such as a transformer, a reactance and the like.
Many examples of methods for controlling the operation of switching devices such as a circuit breakers, disconnectors and the like are reported in the state of the art. So, it is well known that switching devices generally operate in response to commands, either automatic or manual that are sent from a control and monitoring panel. This means that operation can take place in a xe2x80x9crandomxe2x80x9d manner in relation to the parameters of the electrical network (either tri-phase or mono-phase) the switching devices are connected to. Performing closing/opening operation asynchronously in relation to the electrical network parameters, generates very often electrical transients and/or electrical arcing phenomena, such as the so called xe2x80x9cpre-strikesxe2x80x9d or xe2x80x9cre-strikesxe2x80x9d, that are generally extremely damaging.
These undesired effects are obviously present when a switching device is disposed on a electrical network connected to an inductive component. After an opening operation is performed, a residual flux is present in the magnetic core of the inductive component. The quantity of residual flux depends on the hysteresis cycle, which characterises the magnetic core of the inductive component. Unfortunately, said quantity is not constant but it varies due to various reasons such as, for example, the presence of stray capacitances. This fact implies that the quantity of residual flux still present in the magnetic core can be unknown. Therefore, the subsequent closing operation, with an xe2x80x9casynchronousxe2x80x9d switching device, can be performed in relation to a magnetic state, which can be very different from the one actually present in the magnetic component. So, high current transients (called xe2x80x9cin-rush currentsxe2x80x9d), which are characterised by very high magnetisation currents (ten-twenty times the rated current) with strong harmonic content, can arise. The presence of high current transients causes power peaks, the management of which is very difficult to perform. So, for example, in low power networks it is necessary to provide for many generators in parallel, in order to avoid network instability problems. Also in emergency network it is necessary to connect the present transformers once at time, waiting often for the lowering of the current transients. In secondary distribution networks many problems can arise due to the failure of protection fuses. Moreover the presence of a strong harmonic content, particularly of the 2nd order, leads to a lower Power Quality (PQ) of the whole network. The presence of a strong harmonic content can also arise problems in the management of the network protection.
In order to minimise these damaging transient effects, many methods of the state of the art provide for performing the electrical operation in a xe2x80x9csynchronousxe2x80x9d manner. The operation is then performed in relation to the network parameters, which is to say to determine, for example in the case of a three-phase network, sequences of optima operating moments. Each of these sequences, hereinafter defined as an xe2x80x9coperation sequencexe2x80x9d, is a sequence of moments in which voltage and/or current of each phase reaches desired values. Each operation sequence can be performed independently for each pole of the switching device.
In case of switching devices connected to inductive components, solutions are known in which the choice of an optimal operation sequence is actually pre-defined.
For example, U.S. Pat. No. 5,119,260 offers a choice of the optimal operation in relation to the opening operation only. The moments, which are out of phase by a fixed amount of time in relation to the passage through the current zero, are pre-set as optimal. This situation is entirely unsuitable for other types of operations, such as the closing operation.
Other solutions allow one to choose the optimal operation sequence in a relation to the type of operation only without taking the load into account. For example, U.S. Pat. No. 5,361,184 takes the current zeros as the optimal moments for opening operations and the voltage zeros as the optimal moments for closing operations, irrespective of the type of load.
Also this type of solutions is unsuitable because it does not take in account the specific problems arising when an inductive component is connected to the network.
Other methods propose as optima for a closing operation sequence, the moments in which the voltage reaches its maximum. Even if this solution represents a technical progress in relation to the previous solutions described, is unsuitable and unable to drastically reduce the current transients. In fact, the mentioned variations of the inductive flux are not taken into account.
In conclusion, one may assert that the known solutions of the state of the art, while achieving the task for which they were designed, do not guarantee the effective reduction of the current transients when switching device operations are performed in presence of inductive components.
The aim of the present invention is to provide a method for controlling the opening/closing operations of a switching device, disposed on a electrical network connected to an inductive component, which allows to reduce drastically the mentioned transient effects.
As part of this task, one object of the present invention is to provide a method, which allows reducing the so called xe2x80x9cin-rush currentsxe2x80x9d to negligible values if compared with the rated current, so as to improve the power quality of the electrical network and to avoid other mentioned problems.
Another object of the present invention is to provide a method, which allows drastically reducing the low order current harmonics, so as to improve the functionality and management of the network protection devices.
A further object of the present invention is to provide a method, which can be easily realised using processing systems of common use.
Thus, the present invention provides a method for controlling the closing operation of a switching device, said switching device being disposed on a electrical network, said electrical network being connected to an inductive component, comprising the following steps:
a) acquiring first data related to the electrical characteristics of said inductive component and said electrical network;
b) based on said first data, selecting, on predefined first storage means containing predefined opening and closing operation sequences, a first predefined opening operation sequence;
c) based on said first predefined opening operation sequence, selecting, on said first storage means, a first predefined closing operation sequence.